1. Field of the Invention
The present invention generally relates to ink jet printhead apparatus and, more particularly, to an ink jet printhead having thermoelectric temperature control means incorporated therein for the selective heating or cooling of the printhead.
2. Description of Related Art
A piezoelectrically actuated drop-on-demand ink jet printhead is a relatively small device used to selectively eject tiny ink droplets onto a paper sheet operatively fed through a printer, in which the printhead is incorporated, to thereby form from the ejected ink droplets selected text and/or graphics on the sheet. In one representative configuration thereof, an ink jet printhead has a horizontally spaced parallel array of internal ink-receiving channels. These internal channels are covered at their front ends by a plate member through which a spaced series of small ink discharge orifices are formed. Each channel opens outwardly through a different one of the spaced orifices.
A spaced series of internal piezoelectric sidewall portions of the printhead body separate and laterally bound the channels along their lengths. To eject an ink droplet through a selected one of the discharge orifices, the two printhead sidewall portions that laterally bound the channel associated with the selected orifice are piezoelectrically deflected into the channel and then returned to their normal undeflected positions. The driven inward deflection of the opposite channel wall portions increases the pressure of the ink within the channel sufficiently to initiate the ejection of a small quantity of ink, in droplet form, outwardly through the discharge orifice.
A drop-on-demand ink jet printhead such as that described herein could be further enhanced if provided with the ability to control its temperature, particularly if the printhead could both selectively raise and lower its operating temperature. More specifically, both the active piezoelectric material used to form sidewall actuators for the ink-carrying channels of the printhead, as well as the ink which fills those channels, are sensitive to temperature changes. Specifically, performance of the piezoelectric material, i.e. the extent to which the piezoelectric material deforms in response to the application of a voltage thereto, will begin to vary if the temperature of the piezoelectric material strays outside of a preferred range. This causes the magnitude of the pressure pulse imparted to the ink contained in an actuated channel to vary from that expected. As the size of the droplet ejected from an actuated channel will vary depending on the magnitude of the pressure pulse, spot size, i.e. the size of an ink spot produced when the ejected droplet strikes a physical medium such as a sheet of paper, will become unpredictable. As a result, the quality of the representation produced using the above described printing process will be degraded.
Similarly, variations in printhead temperature may cause problems with the ink which fills the channels of the printhead. For example, it is contemplated that, in one application of the disclosed printhead, phase change ink will be used. Typically, phase change ink is solid at room temperature. As such, it is necessary that it be heated above room temperature before it will flow effectively from the supply source to and through the small channels within the printhead. Furthermore, once supplied to the channels, it must be maintained at the elevated temperature to prevent a partial or total return to the solid state and thereby ensure that the printhead will be able to properly eject a droplet of ink upon demand. For example, should the temperature of the ink drop such that a solid particle of ink is formed, that particle could adversely affect operation of the printhead in many of the same ways that foreign particulate matter affects the printhead, for example, by clogging an ink ejection orifice associated with a channel of the printhead. Even minor variations in temperature could potentially result in a change in the viscosity of the ink sufficient to cause a modification of the operating characteristics of the printhead.
While a device or system, either separate to or incorporated therewith, for lowering the temperature of a drop-on-demand ink jet printhead is not known by us, several heating systems which elevate temperature and are suitable for use with ink jet printhead apparatus are known. In many of these configurations, the ink supply and the printhead itself are separate units. In these configurations, the ink is heated by an external heating apparatus positioned on both the ink supply source and the printhead itself. The ink, most commonly, the aforementioned phase change ink, is heated sufficiently to achieve a liquefied ink that will easily flow through the entire printhead ink distribution system. After the ink has been sufficiently heated at the supply source, the ink is transferred from the supply source to the printhead that is heated by an external heating apparatus. The heated printhead maintains the ink's liquidity so that it will flow freely though the small printhead channels and orifices. The ink is then ejected from the printhead onto the paper. In those configurations where the ink supply source and the printhead are one unit, the entire unit is heated by a single external heating apparatus.
In view of the foregoing it can readily be seen that it would be desirable to provide a drop-on-demand ink jet printhead having temperature control means configured to both selectively heat and cool the printhead and a method of manufacturing a printhead having the aforementioned temperature control means incorporated therewith.
It is accordingly an object of the present invention to provide such a printhead and associated method of manufacture.